1. Field of the Invention
The present invention relates to a surface light source using light emitting diodes (LEDs) and having improved light emission quality and a backlight unit for a liquid crystal display (LCD) having the surface light source, and more particularly, to a surface light source that can provide uniform white light by eliminating color stain phenomenon by arranging LEDs such that two adjacent green LEDs can be surrounded by red and/or blue LEDs.
2. Description of the Related Art
A cold cathode fluorescent lamp (CCFL) is widely used as a light source of a conventional backlight for an LCD. Since the CCFL uses mercury gas, it may cause the environmental pollution. Furthermore, the CCFL has a relatively slow response time and a relatively low color reproduction. In addition, the CCFL is not proper to reduce the weight, thickness and overall volume of an LCD panel to which it is applied.
However, LEDs are eco-oriented and have a response time of several nano seconds, thereby being effect for a video signal stream and enabling an impulsive driving. Furthermore, the LEDs have 100% color reproduction and can properly vary luminance and color temperature by adjusting a quantity of light emitted from red, green and blue LEDs. In addition, the LEDs are proper to reduce the weight, thickness and overall volume of the LCD panel. Therefore, in recent years, they have been widely used as a light source of a backlight unit for the LCD.
The LCD backlight employing the LEDs can be classified into an edge type backlight and a direct type backlight according to positions of the light source. In the edge type backlight, the light source is positioned at a side and emits light toward a front surface of the LCD panel using a light guide plate. In the direct type backlight, the light source is a surface light source placed under the LCD panel and having a surface area almost identical to that of the LCD panel and directly emits light toward the front surface of the LCD panel.
In order to generate white light by mixing red, green and blue colors, as shown in FIG. 1B, a convention direct type LCD panel uses a surface light source 300 having a plurality of 2×2 LED matrixes 305 that are arranged along rows and columns. As shown in FIG. 1A, each of the 2×2 LED matrixes 305 includes red and blue LEDs arranged in a first diagonal direction and two green LEDs arranged in a second diagonal direction.
In FIGS. 1A and 1B, reference symbols “R,” “G” and “B” represent the red LED, the green LED, and the blue LED, respectively. Generally, in order to generate the white light, two green LEDs, one red LED and one blue LED are used.
Referring to FIG. 1B, the red light, the green light and the blue light are effectively mixed with each other at a central portion of the surface light source 300, thereby uniformly generating the white light. However, since the blue or red LEDs cannot be properly arranged at edge portions 311 and 312, red color light or blue color light is emitted from the edge portions 311 and 312.
That is, only the red LEDs (R) and the green LEDs (G) are alternately arranged at the edge portion 311, but no blue LED (B) is arranged at the edge portion 311. Also, the red color light is emitted from the edge portion 311. Only the blue LEDs (B) and the green LEDs (G) are alternately arranged at the edge portion 312, but no red LED (R) is arranged at the edge portion 312. Also, the blue color light is emitted from the edge portion 312.
That is, the surface light source 300 using the above-described LED arrangement cannot uniformly emit the white light at the edge portions 311 and 312 thereof.
FIG. 2 is a schematic view of a conventional surface light source for solving the above-described problems. According to this conventional surface light source 400, blue, green, green, and red LEDs are repeatedly arranged in this order along a first row 410. Also, green, red, blue, and green LEDs are repeatedly arranged in this order along a second row 420, and green, blue, red, and green LEDs are repeatedly arranged in this order along a third row 430. In addition, red, green, green, and blue LEDs are repeatedly arranged in this order along a fourth row.
In the conventional surface light source 400, since the green, red, blue, and green LEDs are provided at edge portions 411 and 412, uniform white light can be emitted from the edge portions 411 and 412. However, since there is a region 415 where four green LEDs are consecutively arranged at a central portion of the surface light source 400, green color light is emitted from the region 415. This causes the color stain of the LCD panel.
An optical simulation was performed for the conventional surface light source 400. As a result, it can be noted that the conventional light source has a color coordinate distribution 450 as shown in FIG. 3. The color coordinate distribution 450 shows that a green color is strongly distributed as compared with a normal color distribution illustrated in FIG. 6 and a color resolution of a human being illustrated in FIG. 7. That is, referring to the color coordinate distribution 450, it can be noted that X-axis value is inversely proportional to Y-axis value. That is, the distribution of Y-axis is low when the distribution of an X-axis is high, or the distribution of Y-axis is high when the distribution of X-axis is low. When comparing this distribution with a normal color distribution illustrated in FIG. 6, the green color distribution is higher than that in FIG. 6. The color distribution strays off from a region (i.e., an oval region 462) that is similar to the color detection level of the human being. That is, the color distribution is included in the outer oval region 464 such that the human being recognizes other colors rather than the white light.
Therefore, the conventional surface light source 400 and the backlight unit using the conventional surface light source 400 cannot provide a uniform white light distribution due to the color stain.